Studies this year have centered about the use of a newly developed antiprogestational compound, RU 486, as a potential contraceptive agent in women. Available contraceptive agents for women are either incompletely effective or have hazardous side effects associated with their use. Thus, new agents are constantly being sought. RU 468 is a potent antiprogestational drug that should theoretically interrupt the effect of progesterone on the endometrium and thus block implantation. The feasability of using this drug for this purpose has been examined in the past year. We have established the dose that reliably blocks luteal phase progesterone effects (5 mg/kg) and have shown that, when given on the 26th day of the cycle, the next reproductive cycle is normal. We have used this material prospectively as a contraceptive in a breeding colony of 50 rhesus monkeys. It was completely effective in preventing pregnancy in these animals. Hence, the stage is set for human studies with this promising new agent.